FILE PHOTO: Democratic Republic of Congo's opposition Presidential candidate Moise Katumbi talks to his supporters after leaving the prosecutor's office in Lubumbashi, the capital of Katanga province of the Democratic Republic of Congo, May 11, 2016. REUTERS/Kenny Katombe/File Photo

DAKAR (Reuters) - Congolese opposition leader Moise Katumbi was refused entry into Congo via the country's land border with Zambia on Friday when he tried to return from exile to submit his candidacy for December's presidential election, he said in a tweet.

Katumbi, one of the main rivals of President Joseph Kabila, left Democratic Republic of Congo in May 2016 after he was accused by the government of plotting against him.

The millionaire businessman and former governor of the copper-mining Katanga region was sentenced to three years in prison the following month for real estate fraud. Katumbi denies the charges and says they are aimed at preventing him from contesting the election.

Concluding that he would not be permitted to land his private jet at Kinshasa airport, Katumbi tried to enter at the border between Zambia and Katanga on Friday.

"The regime has forbidden me from landing and barricaded the border," Katumbi tweeted. "My crime? Wanting to enter my country and file my candidacy. In trying to block me, they want to deny the rights of the Congolese to a real election. I will fight!"

Footage from his campaign team sent to Reuters TV showed him with a crowd of thousands of supporters at the border town of Kasumbalesa, on the Zambian side.

His treatment contrasts markedly with that of Kabila's other main rival, former warlord and runner-up presidential candidate Jean-Pierre Bemba, who was allowed to land his plane and given police protection for his return to town.

Bemba filed his candidacy on Thursday, an unlikely comeback after his war crimes conviction at the ICC was overturned, freeing him from a decade in prison.

Both pose a real threat to Kabila or his chosen successor in the upcoming election, especially since the president's popularity has waned over the years.

In power since 2001, Kabila has refused to step down since his mandate ran out in 2016. This has raised fears he will defy constitutional term limits and spark a violent backlash by the population and militia groups active in the country's east.